The invention relates to a method for manufacturing a molded article of high density carbon. The article can be pressed from a powder, the un-fired compact is carbonized and then graphitized.
Japanese Patent Document JP 5-221719 (A), discusses a process for manufacturing molded parts from high density carbon. The parts are close to the final desired dimension, have at least one cavity, and are particularly intended for the manufacture of pistons for internal combustion engines. For manufacturing a molded part, an un-fired compact is pressed from a sinterable powder, a powder of a high density and a grain size of an average of 15 .mu.m. The un-fired compact is carbonized by heating to temperatures between 800.degree. C. and 1,200.degree. C. and is then graphitized by heating to temperatures between 2,600.degree. C. and 3,200.degree. C.
To form the un-fired compact, the powder is filled into a pressing mold of a pressing tool, which has a final dome-shaped die. The mold piece provided for forming the cavity in the part is made of a rigid material, particularly of metal. The pressed, un-fired compact is then removed from the pressing tool. To form the finished part, the un-fired compact is then carbonized and subsequently graphitized. To simplify removal of the un-fired compact from the pressing tool, the die has a slight conical shape, which opens up at the end the compact is removed from.
However, the molded parts produced in the manner according to the Japanese document have cracks in the edge areas. Therefore, on the edges, the parts must be made to a high over-dimension compared to the final desired dimension. Thus, in the manufacturing of the final article, increased machine work as well as time and cost expenditures are required. In addition, because of a tendency to break apart, the number of rejects produced from this process is very high, despite the over-dimension used.
To avoid this disadvantage, other documents of the art suggest using a die made of a rubber-elastic material and/or using a non-sinterable powder for producing the un-fired compact. The other working steps, such as the pressing, carbonizing and graphitizing, remain the same. The molded parts obtained by means of this process remain largely free of cracks. However, the time and energy requirement, approximately 20 days to one month for carbonizing the un-fired compact, is very high.
The present invention provides a cost effective process to manufacture articles made of a high density carbon where the manufacturing and finishing is more reasonable with respect to cost. The invention provides a method for manufacturing a high density carbon article that comprises first compacting a carbon powder into a rigid die. The carbon powder comprises a binderless, self-sintering, fine-grained carbon of a density of more than about 1 g/cm.sup.3 and having an average grain size of between about 5 to about 20 .mu.m. Preferably, the carbon content of the carbon powder is greater than about 90% by mass, the carbon powder comprises a mesophase carbon, and has a density of about 1.4 g/cm.sup.3.
Generally, the compacting of the powder into a rigid die occurs at a pressure of between about 50 to about 150 MPa, preferably 100 MPa, the pressure being preferably released slowly. A precompaction step may also be included before pressing the powder into the die. The precompaction step preferably involves shaking the die but may also include uniaxially precompacting into the rigid die and subsequently isostatically pressing or cold-isostatically pressing the powder. Uniaxial compacting and isostatic pressing or cold-isostatic pressing can also be employed for the compacting process. Compacting the powder under pressure forms a compact, or an un-fired molded product.
After the compact is formed, it is subjected to a carbonizing process. Generally, carbonizing involves heating in an inert atmosphere to a temperature of between about 500 to about 700.degree. C. Following the heating to about 500 to about 700 .degree. C., the temperature is raised to a maximum of between about 800 to about 1,200.degree. C. to form a carbonized compact. The inert atmosphere can be a noble gas, such as the preferred argon, nitrogen, or any other suitable inert gas. Also, the carbonizing process preferably employs a temperature gradient to raise the temperature of the compact. That temperature gradient may be interrupted by one or more holding periods or pauses.
Following the carbonizing, the compact is graphitized. Generally, a graphitizing temperature of between about 2,000 to about 3,000.degree. C. is used in an inert atmosphere. Again, temperature gradients, with or without pauses, may be employed.
As understood in the art, the temperature of the heating chamber will be roughly equivalent to the temperature of the compact. As used herein, the temperature of the compact can be the actual temperature of the compact, the temperature of the heating chamber, or the temperature that the heating chamber is set to.
In specific embodiments, the temperature gradients in the carbonizing step is between about 5 to about 20 K/min., especially for raising the temperature of the compact to a maximum of between about 800 to about 1,200.degree. C. or until the compact reaches a temperature of about 150 to about 200.degree. C., or a gradient of between about 0.05 to about 0.5 K/min., preferably 0.1 K/min.
During the graphitizing step, the heating rate of between about 0.05 to about 1 K/min. is preferably used until the compact reaches the temperature of between about 1,400 to about 1,800.degree. C., and thereafter a heating rate of between about 2 to about 20 K/min, preferably 5 K/min., is used. Also during the graphitizing, a gradient up to a first temperature of between 1,400 and 1,800, particularly 1,600.degree. C., at between about 0.05 and about 1 K/min., preferably approximately 0.2 K/min, is used. After reaching that first temperature, a heating rate of between about 2 to about 20, preferably about 5 K/min, is selected.
In addition, the holding period or pause in the gradients can be one where the compact is held at a temperature of between about 500 to about 700.degree. C. for a period shorter than about two hours, or where at least one holding period occurs at approximately 1,600, 1,900, or 2,500.degree. C. in the respective carbonizing or graphitizing steps.
Also, the carbonizing process may optionally include embedding the compact in a heat-conducting material. Various appropriate heat-conducting materials are known in the art that will not disturb the form of the article and will allow the carbonizing process to proceed. In particular, a heat-conducting material may comprise boron nitride and/or is a loose powder bulk material. In addition, the carbonized compact may optionally be cooled before the graphitizing process begins. The degree of cooling is not particularly critical.
The invention also provides articles produced from any of the methods disclosed herein. Numerous applications for high density carbon articles exist in industry. In a preferred embodiment, the article produced can be used as a component in an internal combustion engine or a crucible. In a particularly preferred embodiment, the article is designed for use as a piston in an internal combustion engine, especially an at least partially hollow piston. In such a case, and in other cases where an article having a cavity is desired, the rigid die employed comprises a mold part designed to form the cavity or hollow part of the article. The mold, generally, is comprised of a rigid metal. The invention is not limited to the use of any particular mold or the production of any particular article. However, preferred molds and considerations for the optimum design of a mold are discussed below or are available to those skilled in the art.
For example, various homogeneous, rigid, and externally polished dies, molds, and cores made of steel are known in the art. These dies can be used to form the desired interior contour of the compact, but previously lead to considerable crack formation. However, an article manufactured according to the method of the invention has a surprisingly negligible amount of cracks and likewise negligible bubbles. For this reason, the dimensions of the articles manufactured, particularly on the edges, can better approach the desired finished dimensions. The reduced deviation of the molded article from the final, desired dimensions results in a shorter machining time and therefore lower cost. In addition, it is surprising that as a result of the invention, the time for carbonizing the pressed un-fired compact can be reduced to approximately 5 days. Thus, the energy consumption and also the costs for manufacturing the article can be dramatically reduced.
Generally, the invention involves pressing binderless, small-grained carbon powders into a non-elastic die and a subsequent temperature treatment, to produce an isotropic article of a high-strength graphite, which is close to the final desired form. Various degrees of closeness to the final desired form may be possible. However, the invention reduces the time-consuming and cost-intensive finishing operations required of prior methods. A suitable powder selection, shaping, and implementation of the process permit the manufacture of homogeneous, crack-free and bubble-free components made of graphite. Thus, depending on the component desired, various permutations in the carbon powders, mold, temperatures used in the steps of the method can be employed by those skilled in the art. Some examples are described below or can be taken from the German priority document. However, the invention is not limited to any particular examples.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example, and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.
As the staring material, binderless, fine-grained carbon powders, particularly carbon mesophases of a powder density of more than 1.4 g/cm.sup.3, are used. The powder is self-sintering and is processed without any additional additives or binding agents. The average grain sizes of the powders used are between 5 and 20 .mu.m. In addition, powders of high carbon contents of approximately 90% by weight are suitable because they result in a high carbon yield after carbonizing.
Pressing the un-fired compact close to the final form may take place either in a cold isostatic press or in a uniaxial press. The cold-isostatic forming tool used, into which the powder is filled, consists of a unvulcanized rubber-type material, such as silicon rubber or a PU-foam material, with a wall thickness of between 5 and 15 mm. A die made of a rigid, non-elastic material is added to the center of these cylindrical forms in that the die is slightly sunk into the bottom of the rubber-type form and cannot slip when the form tool is filled with powder and during the pressing. The die used here results in a cavity in the final product. So that the wall thickness of the un-fired compact to be pressed is approximately the same everywhere, the shape of the forming tool is constructed to be approximately equidistant to the wall of the die.
An advantageous design of the die was found to be a slightly conical shape because, by means of the pressing operation, the powder is pressed onto the die and adheres slightly. Using even a slight conical form or slope in the die significantly simplifies the removal of the un-fired compact after the pressing. This applies particularly to dies which have no rotational symmetry.
The powder is filled into the remaining cavity between the die and the rubber-type form and is slightly compressed manually or by shaking in order to achieve a good filling ratio. The filled mold is closed by means of a lid, which also consists of a rubber-type material, and has the same wall thickness as the rest of the forming tool.
During cold isostatic pressing, the pressure used, as a function of the grain size and of the density of the powder used, is between 50 and 150 MPa. The maximal pressure is maintained for up to 10 minutes.
During the pressing process, it is advantageous to control the pressure reduction from approximately 8 MPa to the normal pressure in order to avoid pressing tensions in the component. During the pressing, the die remains in the center of the rubber-type form filled with powder and is also pressed and supplies a true-to-form interior contour of the cavity. By means of this operation, a clean and smooth contour and interior surface of the pressed compact is obtained. In contrast, in the case of the cold isostatic pressing, the exterior walls of the pressed un-fired compact are slightly curved because, during the pressing onto the non-deformable core, the powder flows plastically into the corners of the rubber-type form where the pressing forces acting upon the form are not so strong.
In order to also construct the outer contour close to the final form and for industrial scale applications, it is advantageous to press the un-fired compact uniaxially. In the case of a uniaxial pressing in a rigid cylinder and when a rigid core is used, not only the interior contour but also the exterior contour can be produced close to the final shape. The wall surfaces produced are smooth, which completely eliminates exterior finishing of the component.
The contact pressure is very dependent on the desired component and is higher than 1 MPa. The pressing mold consists of a metal, such as aluminum, whose surface is coated with a hard material and/or anodized. It may be required to isostatically recompact the uniaxial compacts, for example, by welding and evacuating in a plastic foil. As a result, the exterior contour and the interior contour will change very little, if at all. The un-fired compacts so far produced according to the method of the invention have bottom and wall thicknesses of more than 5 mm.
For the carbonizing, it is advantageous for the un-fired compact to be charged into a crucible, which contains a heat-conducting material, such as boron nitride. The un-fired compact is embedded completely in the loose powder bulk of the heat-conducting material. During the temperature treatment, the material surrounding the mold-part must not react with the product, must not adhere, and must itself not be sinterable. Also, it must be easily removable after the carbonizing.
The temperature treatment is carried out in an inert gas atmosphere, for example, under nitrogen. Up to a temperature of approximately 200.degree. C., a fast heating rate of approximately 10 K/min. can be used. Starting at 200.degree. C., the first compacting step and shrinkage process in the pressed un-fired compact will start. In this case, a slow heating rate of approximately 0.1 K/min. will preferably be selected. In the temperature range between 500 and 700.degree. C., an intermediate holding time expediently takes place so that the material will have time for the chemical reactions to occur and the pyrolysis gas to be released to diffuse to the component surface. Subsequently, a faster heating rate of approximately 0.2 K/min. is used up to the end temperature of 1,000.degree. C. The whole process time amounts to, for example, approximately 4 days.
After the carbonizing, a compact, crack-free body is obtained which, in comparison to the un-fired condition, has shrunk by approximately 30% in volume. The compact carbonized crucible form consists of a partially crystalline, partially graphitized carbon material and is hard and brittle in this condition. The structure of the carbonized compact is homogeneous. The interior contours, which are caused by the pressing operation of the die or onto the core, are obtained in a true-to-size manner.
For the graphitizing process, it is not absolutely necessary to burn the carbonized crucible in a powder bed. The graphitizing of the carbonized un-fired compacts takes place in an inert atmosphere; for example, using argon, and is conducted at temperatures of from 2,000 to 3,000.degree. C., particularly above 2,500.degree. C. The carbonized un-fired compact is taken out of the carbonizing furnace, and, in the cooled condition, is charged into a graphitizing furnace. As a result, contaminations are avoided during the subsequent graphitizing because contamination degassing from the un-fired compact during the carbonizing will remain in the carbonizing furnace. The graphitizing furnace is heated to approximately 1,000.degree. C. with a temperature gradient of approximately 10 K/min. At temperatures of between 1,000.degree. C. and approximately 1,600.degree. C., a heating rate of approximately 0.2.degree. K./min. is selected. Up to the desired graphitizing temperature, heating rates of up to 5 K/min. can be used. Advantageously, pauses are made at temperatures of 1,600, 1,900, and 2,500.degree. C., which are used for improving time-dependent diffusion within the interior of the component, which is advantageous for achieving good material properties of the end product.
During the graphitizing, mainly compacting processes occur in the body volume. In addition, the crystallinity of the carbons will rise considerably and it becomes increasingly graphitic in its properties.
An article produced by means of the method according to the invention is crack-free and homogeneous. The molded part has a homogeneous fine-grained structure and therefore has excellent physical and mechanical properties for a graphite. The surfaces which faced the core during the pressing are smooth. The volume shrinkage with respect to the un-fired compact is 40-50% by volume.